1. Field of the Invention
The present invention relates to a polycarbonate copolymer and a process for producing the same. More particularly, it pertains to polycarbonates excellent in heat resistance and impact resistance, and suitably used for optical parts, mechanical parts, electric and electronic parts, and automotive parts; among said polycarbonate copolymers, a polycarbonate particularly excellent in thermostability at molding and free from problems such as rust on the die (die rust) at molding; and further a process for efficiently producing these polycarbonate copolymers.
2. Description of the Related Arts
It has been known that a polycarbonate copolymer containing 9,9-bis(4-hydroxyphenyl)fluorene has a high heat resistance (U.S. Pat. No. 3,546,165).
However, polycarbonates containing 9,9-bis(4-hydroxyphenyl) fluorene are low in impact resistance, and it is eagerly wished for their improvement.
As a polycarbonate having an improved impact resistance, a block copolymer with siloxane, for instance, is known (J. Appl. Polym. Sci. Vol.20, P-3275, 1976). Said polycarbonate copolymer with siloxane is, however, expensive and not so useful in industrial use, and its improvement over previous polycarbonates is not so remarkable.
On the other hand, when a polycarbonate copolymer containing 9,9-bis(4-hydroxyphenyl)fluorene is produced by the interfacial polycondensation, usually it is not preferable to use a sodium hydroxide aqueous solution and methylene chloride. This is because 9,9-bis(4-hydroxyphenyl)fluorene is not so soluble in any of sodium hydroxide aqueous solution and methylene chloride, and therefore the 9,9-bis(4-hydroxyphenyl)fluorene is hard to be polymerized. Accordingly, such a method brings a poor efficiency in production. Moreover, with the polycarbonate obtained by that method, the reaction does not proceed sufficiently so that phenolic terminal hydroxyl groups and unreacted chloroformate groups are liable to remain. Therefore, the thermostability at molding of said polycarbonate is very poor.
On the other hand, attempts have been made to use 1,2-dichloroethane to dissolve 9,9-bis(4-hydroxyphenyl)fluorene well (U.S. Pat. No. 3,546,165, and the Japanese Patent Application Laid-Open No. 182336/1989 ) or to use chloroform for producing the above-mentioned copolymer. These solvents are, however, industrially expensive, and remain in the polymer after the polymer is isolated, causing thermal deterioration of the polymer or die rust at molding.